The second constituent, α Centauri _B_, is considerably redder than the sun, its surface-temperature being only about 4400 degrees against the sun’s 6000 degrees or so. It has 97 per cent. of the sun’s weight, but only about a third of its luminosity. Yet, as a consequence of its low temperature, it needs 50 per cent. more area than the sun to discharge a third of the sun’s radiation; this makes its diameter 22 per cent. greater than that of the sun. α Centauri _A_ and α Centauri _B_ together form a visual binary, the two components revolving about one another in a period of 79 years. Neither of these two constituents is very dissimilar from the sun, but the third star of the system, Proxima Centauri, is of an altogether different type. It is red in colour, with a surface-temperature of only about 3000 degrees. It is exceedingly dim, emitting only a ten-thousandth part as much light as the sun, and has only a fourteenth part of the sun’s diameter. Its weight is unknown. [Illustration: Fig. 17. The System of α Centauri, with the Sun for comparison.] The sizes of the three stars of this system, with that of the sun for comparison, are shewn in fig. 17. _Munich 15040._ This is a single faint star about which little is known. Its surface is red, with a temperature probably little above 2500 degrees, and it emits only ¹/₂₅₀₀th of the light of the sun. _Wolf 359._ This is the faintest star yet discovered, but beyond this very little is known about it. It is red in colour and emits only about 1/50,000th of the light of the sun. _Lalande 21185._ Another faint red star, emitting ¹/₂₀₀th of the light of the sun. _The System of Sirius._ This consists of two very dissimilar stars, there being some suspicion that a third also may exist. The principal star, Sirius _A_, which appears as the brightest star in the sky (the Dog-star), is white in colour and has a surface-temperature of about 11,000 degrees. As this is nearly twice the sun’s temperature, Sirius _A_ emits nearly 16 times as much radiation per square inch as the sun. Its luminosity is about 26 times that of the sun, and this requires the star’s diameter to be 58 per cent. greater than that of the sun. It has nearly four times the sun’s volume, but only 2·45 times its weight, so that matter is not as closely packed in Sirius _A_ as in the sun. An average cubic metre contains 1·42 ton in the sun, but only 0·93 ton in Sirius _A_. [Illustration: Fig. 18. The System of Sirius, with the Sun for comparison.]